The present invention relates to laser drive circuits and more particularly to laser drive circuits which incorporate vertical metal-on-silicon (VMOS) transistors therein.
High-power injection lasers, also known as junction-diode lasers, are customarily pulsed at a low duty factor, normally less than 1%, in order to avoid large internal power dissipation. The pulsewidths of such pulses are on the order of 10-200 nanoseconds, and have a large current magnitude, on the order of 20-50 amperes. The characteristics of the injection laser are such that it is most desirable to drive the laser with current pulses whose rise and decay times are very short. Various methods of generating such pulses have been devised, but each one has drawbacks in terms of its operating characteristics.
Electromechanical switches have been employed to pulse injection lasers, but such switches are unacceptable due to low speed, contact bounce, and shock immunity problems. A common-emitter bipolar junction transistor switch has also been employed. However, because of the high peak current and off-state collector-to-emitter voltages associated with this type of circuit, a power transistor is required to drive the laser. One typical common-emitter bipolar junction transistor is rated at 30 amperes, 100 volts and 150 watts, and can saturate to a V.sub.CE(Sat) =1.7 volts. However, this transistor has a current-gain cutoff (f.sub.t) of only 30 MH.sub.z and a collector-to-base capacitance of about 600 pF. Therefore, a circuit employing this transistor would have a relatively low operating speed, since the circuitry driving the transistor would be severely stressed during operation. Also, both the turn-on and turn-off capabilities of this type of switching device are unacceptable, due to slow rise and decay times.
Four layer semiconductor switches have gained considerable acceptance for the laser-firing function. Unfortunately, such devices cannot be turned off except by causing the current flowing through them to become zero. This may be accomplished by employing a second switch in series to shut off the flow of charge, but this also reduces the operating speed of the device. Also, the charge storage element may be drained of energy, thereby stopping the flow of charge. The inability to adequately turn the device off on command as well as its relatively slow turn-on characteristic reduce the value of this type of switch for use in generating fast laser firing pulses.
A bipolar junction transistor used in a common-emitter switch configuration may be employed to turn on a current pulse capable of firing a high-power laser. The switch is operated at a collector voltage well above its rating, and a turn-on pulse applied to the base can cause an abrupt collapse of the collector-to-emitter path, commonly known as an avalanche condition, thus allowing the transistor to rapidly turn on current to the laser. A typical bipolar junction transistor avalanches at voltages between 200-300 volts. The turn-off of such a device is usually accomplished by exhausting the energy in a charge storage capacitor similar to the above-mentioned silicon-controlled-rectifier switch. Problems with both types of switches include that of temperature-compensation and thermal drift, as well as inability to turn the switch off on command.
Accordingly, it would be an improvement in the laser drive circuit art to have a circuit which may rapidly and accurately turn current on or off to a laser source in response to a low voltage command signal.
It would also be an improvement in the laser drive circuit art to have a circuit which pulses a laser source in response to low voltage command signals such as may be provided by conventional logic circuits.